Help:Pressure after mixing

Hello sirs, It is once again a basic(?)fluid mechanics doubt.A centrifugal pump delivers 10 units of discharge at 100 units of pressure through a pipe.There is going to be a bypass pipe line attached to the main pipe line branching off for a short distance of say 1m and rejoining to the mainline.A valve placed on the main line downstream of the bypass point is so operated that 90 units of discharge is bypassed and the pressure in the main line is droped to 20 units.

Now the branched pipe line carrying 90 units of discharge (presumably) at 100 units of pressure rejoins to the main line water existing at 20 units of pressure.The mixture will have a discharge of 10 units.

The question is what can be pressure in the pipe line now.Many people swear that it will be 20 units only quoting the electrical anology while I am not able to be convinced of this.Can't the pressure be at a value in between 20 and 100 units of pressure based on the amount of bypass. With warm regards, asok